On Slam's move page, under breeding it doesn't list Marill as a possible option. Considering both Azurill and Azumarill can learn it through breeding I assume Marill can as well.

EDIT: Also I have noticed a similar problem on the Last Resort move page, where Chansey is not listed as a possible one to learn it through breeding, and yet Blissey is. I figured I would just add it here, instead of asking a new question for it, since it is basically the same problem, and no one has responded to this yet.